CHERNOBYL

Positive Void Coefficient

Inside the reactor it now gets hot enough to form steam bubbles. This increases power output because steam absorbs neutrons less effectively than the water it displaces. Positive void coefficient! The nuclear reaction intensifies when the cooling water or moderator boils away. All RBMK reactors at the time of the accident had a dangerously high positive void coefficient—a design well-suited for runaway problems. After the accident, existing RBMK reactors were modified to reduce the void coefficient, but it is still positive.